Sync only a certain tag or folder with iPhone

Recently I gave my wife an iPhone, and would like to put a simple tasks application on it for her, but ideally that can be synced with the web.

A full-blown Toodledo app and account is way too sophisticated for what she needs, but then I thought: if I can sync with her iPhone only tasks that are under a certain tag or folder, that would be ideal (both for syncing and for me to assign her tasks via web or even from my Android).

I don't want to sync all tasks with her iPhone, because a lot have to do with my work and daily things, and that would just get in her way.

Before I commit for the official Toodledo app (or any other app), I would like to find one that can do as I'm saying.